Sometimes descriptor string 0xee is requested.
This is a mechanism used by Microsoft Windows to further identify the USB device.
Instead of stalling, as is the original code, leading to an USB reset, we send an empty packet.
I am not sure if sending an empty string would be better, but an empty packet seems sufficient.
e.g. in CCID mode we need to treat class-specific control requests,
and we want to do this in a way how the CCID code doesn't need to
understand about DFU.
We cannoy simply use the DFU runtime descriptor of the DFU mode, but we
have to use the descriptor of the specific currently-selected runtime
configuration. Let's iterate over the descriptors of a configuration
and find the DFU runtime descriptor in it.
Using the USBDFU_OverrideEnterDFU() function, a board/application can
define extra conditions when the system should boot in DFU mode, even if
it was not explicitly switched to DFU mode from the application.
The app/dfu/main.c uses this mechanism to boot into DFU mode if the
stack + reset vector addresses are not plausible (i.e. some random junk
appears to be flashed in the application partition) or if the user
places a jumper accross the RxD+TxD lines of the debug UART. The idea
is that the system can be recovered by placing this jumper and then
re-installing the application from DFU.
It is utterly unacceptable that Atmel is shipping example code in 2011,
which doesn't support parts that are shipped still in 2016. They would
have had five years to fix their code to implement the chip errata :(
We need to refer to g_dfu as a pointer from all code. In DFU mode, it
gets assigned to the address of _g_dfu, which is placed by the linker
script at the start of RAM. In runtime mode, the pointer is statically
defined to point at the start of RAM. The linker script for the runtime
(dfu environment) is adjusted to save the first 32 bytes for the _g_dfu
structure.
* we now actually route the EP0 control requests in DFU mode to the
correct handler (weak linker symbols are tricky)
* we now actually call code to read/write data from/to RAM/FLASH
This way we can easily check with 'diff' for differences in our code and
Atmel softpack. Also, this layout is more suitable for building various
different firmware images (e.g. factory-test, dfu-loader, main
application) for a variety of different boards (simtrace, owhw, qmod).
